Measurement of blade clearance in all stages of a gas turbine engine, jet engine or any other turbo-machines and turbines is an important step towards reducing maintenance and improving the thermodynamic efficiency and thrust output of the engine. Any gas which travels between the blade tips and the engine shroud represents a significant loss of energy from the system, lowering thrust and requiring the consumption of additional fuel. Thus, it is desirable for these reasons to minimize the clearance of the blades. Doing so, however, runs the risk of a catastrophic failure occurring if the blades impact the engine shroud. It is desirable, therefore, to know how much clearance is present and to utilize that data to maintain a minimal, yet safe, clearance. Once this information is available, it can be applied as the driver for a suite of active control technologies, such as vibration cancellation, adaptive modification of the housing diameter, or emergency shutdown to prevent catastrophic failure.
The patent literature discloses several attempts to measure rotor tip clearance in turbo-machineries. U.S. Pat. No. 4,049,349 to A. J. Wennerstrom discloses a measuring device comprising a pair of optical sensors aimed at the rotating blades, and a single sensor aimed at the rotating shaft. Each sensor comprises a light emitter and a light detector. The pair of optical sensors aiming at the rotating blades is spaced a short distance apart, and their light emitters project light beams at an angle to one another. A digital clock comprising a signal generator producing a stable frequency is used as a timing device for the system. The light beams aimed at the rotating blades are reflected and scattered by passing blades. As a rotating blade intercepts the first light beam, its reflected light starts a counter associated with the digital clock. As this rotating blade intercepts the second light beam from the pair, its reflected light stops the counter. The sensor aiming at the rotating shaft starts and stops another counter recording the number of cycles or pulses from the digital clock, which occurred during one revolution of the shaft. By dividing the number of counts measured by the pair of sensors aiming at the rotating blades by the number of counts during one revolution, the rotor tip clearance is ascertained.
U.S. Pat. No. 4,326,804 to P. W. Mossey discloses another tip clearance measuring device that comprises a single light emitter and a single light receiver. The emitted light impinges the rotating blades at an angle. The reflected light is focused on to a position detector. The tip of the rotating blade reflects light at varying angles as a function of the tip clearance. The angles of the reflected light are detected by the position detector, and the tip clearance is derived from said angles.
U.S. Pat. No. 5,017,796 to H. Makita also discloses a tip clearance measuring device with a single light emitter and a single light receiver. This device has a holding spring that biases a movable focusing lens to focus the emitted light on to the moving blade. The movable lens is adjusted by oscillating movement until the reflected light has a maximum value. The tip clearance is related to the position of the movable lens at the maximum value of the reflected light.
U.S. Pat. No. 4,357,104 to I. Davinson passes light through an astigmatic lens which changes the shape of the beam to measure the tip clearance. U.S. Pat. No. 4,596,460 to I. Davinson uses an optical triangulation technique using a T-shaped optical path to measure tip clearance. U.S. Pat. No. 4,180,329 to J. R. Hildebrand discloses a single blade proximity probe using two light beams having different frequencies. The two light beams are mixed prior to being projected towards the blades, and the reflected signal is subtracted by the frequency of the second light beam.
Commonly-owned U.S. published patent applications US-2010-0177299 A1 and US-2010-0168981 A1 by the present inventor disclose an apparatus and a method for ascertaining a gap between a stationary member and a rotating member. At least a reference light beam and a signal light beam, which have different focal lengths or which diverge/converge at different rates, are fixed to the stationary member and proximate to each other. The beams are projected across a gap between the stationary member and the rotating member toward the rotating member. The reference and signal beams are reflected by the translating member when it intersects the reference and signal beam, and the reflected reference and signal pulses are obtained. The diameters of the reference beam and the signal beams have different diameters at the line of intersection. One or more features of the reflected reference pulse and the reflected signal pulse, such as a rise time of the pulses, a fall time of the pulses, a width of the pulses and a delay between the reflected reference pulse and the reflected signal pulse, among other factors, are obtained. The width of the gap is obtained using at least one of these factors. References US-2010-0177299 A1 and US-2010-0168981 A1 are incorporated herein by reference in their entireties.